HIGH-DUCTILITY, HIGH-STRENGTH ELECTROLYTIC ZINC-BASED COATED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

- JFE Steel Corporation

A high-ductility, high-strength electrolytic zinc-based coated steel sheet includes an electrolytic zinc-based coating on a base steel sheet, in which the base steel sheet has a predetermined composition and a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet.

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Description
CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/030793, filed Aug. 6, 2019, which claims priority to Japanese Patent Application No. 2018-196591, filed Oct. 18, 2018, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high-ductility, high-strength electrolytic zinc-based coated steel sheet and a method for producing the same. More specifically, the present invention relates to a high-ductility, high-strength electrolytic zinc-based coated steel sheet used, for example, for automotive components and a method for producing the same, and in particular, to a high-ductility, high-strength electrolytic zinc-based coated steel sheet excellent in bendability and a method for producing the same.

BACKGROUND OF THE INVENTION

In recent years, efforts have been actively made to reduce the weight of vehicle bodies themselves. The thicknesses of steel sheets used for vehicle bodies have been reduced by increasing the strength of steel sheets. In particular, there have been advances in the use of high-strength steel sheets with 1,320 to 1,470 MPa-grade tensile strength (TS) to vehicle frame components, such as center pillar reinforcements (R/F), bumpers, and impact beam components (hereinafter, also referred to as “components”). Furthermore, from the viewpoint of further reducing the weight of automotive bodies, studies have been conducted on the use of sheets of TS 1,800 MPa (1.8 GPa) or higher grade steels. Additionally, from the viewpoint of workability, there is a growing demand for steel sheets with bendability.

With an increase in the strength of steel sheets, hydrogen embrittlement may occur. In recent years, it has been suggested that plating hinders the release of hydrogen that has entered a steel sheet during the production process of the steel sheet and there is the risk of a decrease in ductility, in particular, local ductility. It has also been suggested that the accumulation of hydrogen in steel around coarse carbides in a surface layer of steel promotes the occurrence of cracking upon working.

For example, Patent Literature 1 provides a high-strength steel sheet having a chemical composition containing C: 0.12% to 0.3%, Si: 0.5% or less, Mn: less than 1.5%, P: 0.02% or less, S: 0.01% or less, Al: 0.15% or less, and N: 0.01% or less, the balance being Fe and incidental impurities, the steel sheet having a single tempered martensite microstructure and a tensile strength of 1.0 to 1.8 GPa.

Patent Literature 2 provides a high-strength steel sheet composed of a steel having a chemical composition containing C: 0.17% to 0.73%, Si: 3.0% or less, Mn: 0.5% to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less, the balance being Fe and incidental impurities, the steel sheet having a good balance between strength and ductility and a tensile strength of 980 MPa to 1.8 GPa, in which the increased strength of the steel sheet is obtained by the use of a martensite microstructure, retained austenite required to provide the TRIP effect is stably provided by the use of upper bainite transformation, and martensite is partially transformed into tempered martensite.

PATENT LITERATURE

  • PTL 1: Japanese Unexamined Patent Application Publication No. 2011-246746
  • PTL 2: Japanese Unexamined Patent Application Publication No. 2010-90475

SUMMARY OF THE INVENTION

In the technique disclosed in Patent Literature 1, although the single tempered martensite microstructure results in excellent strength, inclusions and coarse carbides that promote crack growth cannot be reduced; thus, the steel sheet is not considered to be excellent in bendability.

In the technique disclosed in Patent Literature 2, although there is no description of bendability, austenite having an fcc structure has a larger amount of hydrogen dissolved therein than martensite and bainite having a body-centered cubic (bcc) structure or a body-centered tetragonal (bct) structure; thus, the steel specified in Patent Literature 2, which contains a large amount of austenite, seemingly contains a large amount of diffusible hydrogen therein and is not considered to be excellent in bendability.

Aspects of the present invention aim to provide a high-ductility, high-strength electrolytic zinc-based coated steel sheet having excellent bendability and a method for producing the steel sheet.

In accordance with aspects of the present invention, the term “high-ductility, high-strength” refers to a tensile strength (TS) of 1,320 MPa or more, an elongation (El) of 7.0% or more, and TS×El=12,000 or more. The term “excellent (in) bendability” indicates that limit bending radius/thickness (R/t) is 4.0 or less in a predetermined bending test.

In an electrolytic zinc-based coated steel sheet, a surface of a base steel sheet refers to the interface between the base steel sheet and an electrolytic zinc-based coating.

A region extending from a surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet is also referred to as a “surface layer portion”.

Aspects of the present invention provide a high-ductility, high-strength electrolytic zinc-based coated steel sheet containing a predetermined amount of fine carbides in a surface layer portion to reduce the amount of diffusible hydrogen in steel and thus having excellent bendability, and a method for producing the steel sheet.

Specifically, a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention includes a layer of electrolytic zinc-based coating on a surface of a base steel sheet and has a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet, and the total perimeter of individual carbide particles having an average particle size of 50 nm or less in the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less present in the region is 50 μm/mm2 or more, and the amount of diffusible hydrogen in steel is 0.20 ppm or less by mass, the tensile strength (TS) is 1,320 MPa or more, the elongation (El) is 7.0% or more, TS×El is 12,000 or more, and R/t is 4.0 or less.

The inventors have conducted intensive studies in order to solve the foregoing problems and have found that the amount of diffusible hydrogen in steel needs to be reduced to 0.20 ppm by mass or less in order to obtain excellent bendability. To reduce the amount of diffusible hydrogen in steel, fine carbides serving as hydrogen-trapping sites need to be increased in a surface layer portion of steel. To this end, it is necessary to prevent decarburization. The following have also been found: Decarburization is suppressed by adjusting the component composition of steel and shortening a residence time from the completion of finish rolling to coiling; thus, an electrolytic zinc-based coated steel sheet having excellent bendability is successfully produced. A microstructure mainly containing martensite and bainite results in high ductility and high strength. The outline of aspects of the present invention is described below.

[1] A high-ductility, high-strength electrolytic zinc-based coated steel sheet includes an electrolytic zinc-based coating on a surface of a base steel sheet,

in which the base steel sheet has a component composition containing, on a percent by mass basis,

C: 0.12% or more and 0.40% or less,

Si: 0.001% or more and 2.0% or less,

Mn: 1.7% or more and 5.0% or less,

P: 0.050% or less,

S: 0.0050% or less,

Al: 0.010% or more and 0.20% or less,

N: 0.010% or less, and

Sb: 0.002% or more and 0.10% or less, the balance being Fe and incidental impurities; and

a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet, and the total perimeter of individual carbide particles having an average particle size of 50 nm or less in the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less present in the region is 50 μm/mm2 or more,

in which the amount of diffusible hydrogen in steel is 0.20 ppm or less by mass.

[2] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in [1], the component composition further contains, on a percent by mass basis:

B: 0.0002% or more and less than 0.0035%.

[3] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in [1] or [2], the component composition further contains, on a percent by mass basis, one or two selected from:

Nb: 0.002% or more and 0.08% or less, and

Ti: 0.002% or more and 0.12% or less.

[4] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in any of [1] to [3], the component composition further contains, on a percent by mass basis, one or two selected from:

Cu: 0.005% or more and 1% or less, and

Ni: 0.01% or more and 1% or less.

[5] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in any of [1] to [4], the component composition further contains, on a percent by mass basis, one or two or more selected from:

Cr: 0.01% or more and 1.0% or less,

Mo: 0.01% or more and less than 0.3%,

V: 0.003% or more and 0.5% or less,

Zr: 0.005% or more and 0.2% or less, and

W: 0.005% or more and 0.2% or less.

[6] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in any of [1] to [5], the component composition further contains, on a percent by mass basis, one or two or more selected from:

Ca: 0.0002% or more and 0.0030% or less,

Ce: 0.0002% or more and 0.0030% or less,

La: 0.0002% or more and 0.0030% or less, and

Mg: 0.0002% or more and 0.0030% or less.

[7] In the high-ductility, high-strength electrolytic zinc-based coated steel sheet described in any of [1] to [6], the component composition further contains, on a percent by mass basis:

Sn: 0.002% or more and 0.1% or less.

[8] A method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet includes:

a hot-rolling step of hot-rolling a steel slab having the component composition described in any of [1] to [7] at a slab heating temperature of 1,200° C. or higher and a finish hot-rolling temperature of 840° C. or higher, performing cooling to a primary cooling stop temperature of 700° C. or lower at an average cooling rate of 40° C./s or more in a temperature range of the finish hot-rolling temperature to 700° C., performing cooling at an average cooling rate of 2° C./s or more in a temperature range of the primary cooling stop temperature to 650° C., performing cooling to a coiling temperature of 630° C. or lower, and performing coiling;

an annealing step of heating a steel sheet after the hot-rolling step to an annealing temperature equal to or higher than an AC3 point or performing heating to an annealing temperature equal to or higher than an AC3 point and performing soaking, performing cooling to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./s or more in a temperature range of the annealing temperature to 550° C., and performing holding at a holding temperature in a temperature range of 100° C. to 200° C. for 20 to 1,500 seconds; and a coating treatment step of cooling the steel sheet after the annealing step to room temperature and subjecting the steel sheet to electrolytic zinc-based coating for an electroplating time of 300 seconds or less.

[9] The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet described in [8] further includes, after the hot-rolling step, a cold-rolling step of cold-rolling the steel sheet between the hot-rolling step and the annealing step.

[10] The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet described in [8] or [9] further includes a tempering step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:


(T+273)(log t+4)≤2,700  (1)

where in formula (1), T is a holding temperature (° C.) in the tempering step, and t is the holding time (s) in the tempering step.

Aspects of the present invention provide a high-ductility, high-strength electrolytic zinc-based coated steel sheet having excellent bendability by adjusting the component composition and the production method so as to suppress decarburization in the surface layer portion, increase the amount of fine carbides in the surface layer portion, and reduce the amount of diffusible hydrogen in steel.

The use of the high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention for automotive structural members can achieve both an increase in the strength and an improvement in bendability of automotive steel sheets. In other words, according to aspects of the present invention, the performance of automotive bodies is improved.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The inventors have conducted various studies in order to solve the foregoing problems and have found that a high-ductility, high-strength electrolytic zinc-based coated steel sheet having excellent bendability is obtained, the steel sheet having a predetermined component composition and a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire microstructure of the steel sheet, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet, and the total of the perimeter (total perimeter) of individual carbide particles having an average particle size of 50 nm or less in the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less present in the region is 50 μm/mm2 or more, and the amount of diffusible hydrogen in steel is 0.20 ppm or less by mass. These findings have led to the completion of the present invention.

Embodiments of the present invention will be described below. The present invention is not limited to the embodiments described below.

A high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention includes a layer of electrolytic zinc-based coating on a surface of a steel sheet serving as a base (base steel sheet).

The component composition of the base steel sheet (hereinafter, also referred to simply as a “steel sheet”) according to aspects of the present invention will first be described. In the description of the component composition, each component content is expressed in units of “%” that indicates “% by mass”.

C: 0.12% or More and 0.40% or Less

C is an element that improves hardenability, and is incorporated from the viewpoint of achieving a predetermined area percentage of martensite and/or bainite and increasing the strength of martensite and bainite to ensure TS 1,320 MPa. Finely dispersed carbides trap hydrogen in steel to reduce the amount of diffusible hydrogen in the steel, thereby improving the bendability. When the C content is less than 0.12%, fine carbides in the surface layer portion of the steel cannot be ensured; thus, excellent bendability cannot be maintained. Accordingly, the C content is 0.12% or more. From the viewpoint of achieving higher TS, such as TS 1,470 MPa, the C content is preferably more than 0.16%, more preferably 0.18% or more. When the C content is more than 0.40%, carbides in martensite and bainite coarsen. The presence of the coarse carbides in the surface layer portion causes the coarse carbides to act as the starting points of bent cracks, thereby deteriorating the bendability. Accordingly, the C content is 0.40% or less. The C content is preferably 0.30% or less, more preferably 0.25% or less.

Si: 0.001% or More and 2.0% or Less

Si is an element that contributes to strengthening by solid-solution strengthening. When a steel sheet is held in a temperature range of 200° C. or higher, Si suppresses the excessive formation of coarse carbides to contribute to an improvement in bendability. Si also reduces the segregation of Mn in the middle portion of the sheet in the thickness direction to contribute to the suppression of the formation of MnS. Additionally, Si contributes to the suppression of decarburization and deboronization due to the oxidation of the surface layer portion of the steel sheet during continuous annealing. To sufficiently provide the effects described above, the Si content is 0.001% or more. The Si content is preferably 0.003% or more, more preferably 0.005% or more. An excessively high Si content results in the extension of the segregation in the thickness direction to easily form coarse MnS in the thickness direction, thereby deteriorating the bendability. Additionally, the formation of carbides is suppressed; thus, the absence of fine carbides increases the amount of diffusible hydrogen at the surface layer in the steel, thereby deteriorating the bendability. Accordingly, the Si content is 2.0% or less. The Si content is preferably 1.5% or less, more preferably 1.2% or less.

Mn: 1.7% or More and 5.0% or Less

Mn is incorporated in order to improve the hardenability of the steel and obtain a predetermined area percentage of martensite and/or bainite. A Mn content of less than 1.7% results in the formation of ferrite in the surface layer portion of the steel sheet to decrease the strength. Additionally, the absence of fine carbides in the surface layer portion increases the amount of diffusible hydrogen in the surface layer portion of the steel to deteriorate the bendability. Accordingly, Mn needs to be contained in an amount of 1.7% or more. The Mn content is preferably 2.4% or more, more preferably 2.8% or more. An excessively high Mn content may result in the increase of coarse carbides in the surface layer portion to significantly deteriorate the bendability. Accordingly, the Mn content is 5.0% or less. The Mn content is preferably 4.8% or less, more preferably 4.4% or less.

P: 0.050% or Less

P is an element that strengthens steel. At a high P content, the occurrence of cracking is promoted. Thus, even in the case of a small amount of diffusible hydrogen in the steel, the bendability is significantly deteriorated. Accordingly, the P content is 0.050% or less. The P content is preferably 0.030% or less, more preferably 0.010% or less. The lower limit of the P content is not particularly limited. Currently, the industrially feasible lower limit is about 0.003%.

S: 0.0050% or Less

S significantly adversely affects the bendability through the formation of inclusions, such as MnS, TiS, and Ti(C,S). To reduce the harmful effect of these inclusions, the S content needs to be 0.0050% or less. The S content is preferably 0.0020% or less, more preferably 0.0010% or less, even more preferably 0.0005% or less. The lower limit of the S content is not particularly limited. Currently, the industrially feasible lower limit is about 0.0002%.

Al: 0.010% or More and 0.20% or Less

Al is added in order to sufficiently perform deoxidation to reduce coarse inclusions in the steel. The effect is provided at 0.010% or more. The Al content is preferably 0.015% or more. At an Al content of more than 0.20%, carbides mainly containing Fe, such as cementite, formed during coiling after hot rolling do not easily dissolve in an annealing step; thus, coarse inclusions and coarse carbides are formed to deteriorate the bendability. Accordingly, the Al content is 0.20% or less. The Al content is preferably 0.17% or less, more preferably 0.15% or less.

N: 0.010% or Less

N is an element that forms coarse nitride- and carbonitride-based inclusions, such as TiN, (Nb,Ti) (C,N), AlN, in the steel, and deteriorates the bendability through the formation of these inclusions. To prevent the deterioration of the bendability, the N content needs to be 0.010% or less. The N content is preferably 0.007% or less, more preferably 0.005% or less. The lower limit of the N content is not particularly limited. Currently, the industrially feasible lower limit is about 0.0006%.

Sb: 0.002% or More and 0.10% or Less

Sb suppresses the oxidation and nitriding of the surface layer portion of the steel sheet to suppress decarburization due to the oxidation and nitriding in the surface layer portion of the steel sheet. The suppression of decarburization suppresses the formation of ferrite in the surface layer portion of the steel sheet, thereby contributing to an increase in strength. Additionally, fine carbides can be provided in the surface layer portion of the steel to reduce the amount of diffusible hydrogen in the surface layer portion of the steel. From this point of view, Sb needs to be contained in an amount of 0.002% or more. The Sb content is preferably 0.004% or more, more preferably 0.007% or more. When Sb is contained in an amount of more than 0.10%, Sb segregates at prior y grain boundaries to promote the occurrence of cracking, thereby deteriorating the bendability. Accordingly, the Sb content is 0.10% or less. The Sb content is preferably 0.08% or less, more preferably 0.06% or less.

The steel sheet according to aspects of the present invention has a component composition having the foregoing components, the balance being Fe (iron) and incidental impurities. The steel sheet according to aspects of the present invention preferably has the component composition, having the foregoing components and the balance Fe and incidental impurities. The steel sheet according to aspects of the present invention may further contain the following components as optional components. In the case where the optional components are contained in amounts of less than the lower limits, the components are contained as incidental impurities.

B: 0.0002% or More and Less Than 0.0035%

B is an element that improves the hardenability of steel, and has the advantage that martensite and bainite having predetermined area percentages are formed even in the case of a low Mn content. To provide the effects of B, B is preferably contained in an amount of 0.0002% or more. The B content is more preferably 0.0005% or more, even more preferably 0.0007% or more. From the viewpoint of immobilizing N, B is preferably added in combination with 0.002% or more of Ti. A B content of 0.0035% or more results in a decrease in dissolution rate of cementite during annealing to leave carbides mainly containing Fe, such as undissolved cementite. This leads to the formation of coarse inclusions and carbides, thereby deteriorating the bendability. Accordingly, the B content is preferably less than 0.0035%. The B content is more preferably 0.0030% or less, even more preferably 0.0025% or less.

One or Two Selected from Nb: 0.002% or More and 0.08% or Less and Ti: 0.002% or More and 0.12% or Less

Nb and Ti contribute to an increase in strength through a reduction in the size of prior y grains. Fine Nb and Ti carbides formed serve as hydrogen-trapping sites to reduce the amount of diffusible hydrogen in the steel, thereby improving the bendability. From this point of view, each of Nb and Ti is preferably contained in an amount of 0.002% or more. Each of the Nb content and the Ti content is more preferably 0.003% or more, even more preferably 0.005% or more. When large amounts of Nb and Ti are contained, coarse Nb-based precipitates remaining undissolved, such as NbN, Nb(C,N), and (Nb,Ti) (C,N), and coarse Ti-based precipitates, such as TiN, Ti(C,N), Ti(C,S), and TiS, are increased during heating of the slab in the hot-rolling step to deteriorate the bendability. Accordingly, Nb is preferably contained in an amount of 0.08% or less. The Nb content is more preferably 0.06% or less, even more preferably 0.04% or less. Ti is preferably contained in an amount of 0.12% or less. The Ti content is more preferably 0.10% or less, even more preferably 0.08% or less.

One or two Selected from Cu: 0.005% or More and 1% or Less and Ni: 0.01% or More and 1% or Less

Cu and Ni are effective in improving the corrosion resistance in an environment in which automobiles are used and suppressing hydrogen entry into the steel sheet by allowing corrosion products to cover the surfaces of the steel sheet. From this point of view, Cu is preferably contained in an amount of 0.005% or more. Ni is preferably contained in an amount of 0.01% or more. From the viewpoint of improving the bendability, each of Cu and Ni is more preferably contained in an amount of 0.05% or more, even more preferably 0.08% or more. However, excessively large amounts of Cu and Ni lead to the occurrence of surface defects to deteriorate coatability and chemical conversion treatability. Accordingly, each of the Cu content and the Ni content is preferably 1% or less. Each of the Cu content and the Ni content is more preferably 0.8% or less, even more preferably 0.6% or less. One or Two or More Selected from Cr: 0.01% or More and 1.0% or Less, Mo: 0.01% or More and Less Than 0.3%, V: 0.003% or More and 0.5% or Less, Zr: 0.005% or More and 0.2% or Less, and W: 0.005% or More and 0.2% or Less

Cr, Mo, and V may be incorporated in order to improve the hardenability of steel. To provide the effect, each of Cr and Mo is preferably contained in an amount of 0.01% or more. Each of the Cr content and the Mo content is more preferably 0.02% or more, even more preferably 0.03% or more. V is preferably contained in an amount of 0.003% or more. The V content is more preferably 0.005% or more, even more preferably 0.007% or more. However, an excessively large amount of any of Cr, Mo, and V leads to coarsening of carbides, thereby deteriorating the bendability. Accordingly, the Cr content is preferably 1.0% or less. The Cr content is more preferably 0.4% or less, even more preferably 0.2% or less. The Mo content is preferably less than 0.3%. The Mo content is more preferably 0.2% or less, even more preferably 0.1% or less. The V content is preferably 0.5% or less. The V content is more preferably 0.4% or less, even more preferably 0.3% or less.

Zr and W contribute to an increase in strength through a reduction in the size of prior y grains. From this point of view, each of Zr and W is preferably contained in an amount of 0.005% or more. Each of the Zr content and the W content is more preferably 0.006% or more, even more preferably 0.007% or more. However, when large amounts of Zr and W are contained, coarse precipitates remaining undissolved are increased during heating of the slab in the hot-rolling step to deteriorate the bendability. Accordingly, each of Zr and W is preferably contained in an amount of 0.2% or less. Each of the Zr content and the W content is more preferably 0.15% or less, even more preferably 0.1% or less.

One or Two or More Selected from Ca: 0.0002% or More and 0.0030% or Less, Ce: 0.0002% or More and 0.0030% or Less, La: 0.0002% or More and 0.0030% or Less, and Mg: 0.0002% or More and 0.0030% or Less

Ca, Ce, and La immobilize S in the form of sulfide, serve as hydrogen-trapping sites in steel, and reduce the amount of diffusible hydrogen in the steel to contribute to an improvement in bendability. For this reason, each of the Ca content, the Ce content, and the La content is preferably 0.0002% or more. Each of the Ca content, the Ce content, and the La content is more preferably 0.0003% or more, even more preferably 0.0005% or more. The addition of large amounts of Ca, Ce, and La coarsens sulfides to deteriorate the bendability. Accordingly, each of the Ca content, the Ce content, and the La content is preferably 0.0030% or less. Each of the Ca content, the Ce content, and the La content is more preferably 0.0020% or less, even more preferably 0.0010% or less.

Mg immobilizes 0 in the form of MgO, serves as a hydrogen-trapping site in steel, and reduces the amount of diffusible hydrogen in the steel to contribute to an improvement in bendability. Accordingly, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0003% or more, even more preferably 0.0005% or more. The addition of a large amount of Mg coarsens MgO to deteriorate the bendability. Thus, the Mg content is preferably 0.0030% or less. The Mg content is more preferably 0.0020% or less, even more preferably 0.0010% or less.

Sn: 0.002% or More and 0.1% or Less

Sn suppresses the oxidation and nitriding of the surface layer portion of the steel sheet to suppress decarburization due to the oxidation and nitriding in the surface layer portion of the steel sheet. The suppression of decarburization suppresses the formation of ferrite in the surface layer portion of the steel sheet, thereby contributing to an increase in strength. Additionally, fine carbides can be provided in the surface layer portion of the steel to reduce the amount of diffusible hydrogen in the surface layer portion of the steel. From this point of view, Sn is preferably contained in an amount of 0.002% or more. The Sn content is more preferably 0.003% or more, even more preferably 0.004% or more. When Sn is contained in an amount of more than 0.1%, Sn segregates at prior y grain boundaries to promote the occurrence of cracking, thereby deteriorating the bendability. Accordingly, the Sn is contained in an amount of 0.1% or less. The Sn content is more preferably 0.08% or less, even more preferably 0.06% or less.

Amount of Diffusible Hydrogen in Steel of 0.20 ppm or Less by Mass

The amount of diffusible hydrogen in accordance with aspects of the present invention indicates the cumulative amount of hydrogen released from a heating start temperature (25° C.) to 200° C. when heating is performed at a rate of temperature increase of 200° C./h with a thermal desorption spectroscopy system immediately after removal of the coating from the electrolytic zinc-based coated steel sheet. When the amount of diffusible hydrogen in the steel is more than 0.20 ppm by mass, cracking is promoted during bending to deteriorate the bendability. Accordingly, the amount of diffusible hydrogen in the steel is 0.20 ppm or less by mass. The amount of diffusible hydrogen in the steel is preferably 0.17 ppm or less by mass, more preferably 0.13 ppm or less by mass. The lower limit of the amount of diffusible hydrogen in the steel is not particularly limited and may be 0 ppm by mass. As the value of the amount of diffusible hydrogen in the steel, a value obtained by a measurement method described in Examples is used. In accordance with aspects of the present invention, the amount of diffusible hydrogen in the steel needs to be 0.20 ppm or less by mass before forming or welding the steel sheet. Regarding a product (member) after forming or welding the steel sheet, in the case where a sample is cut out from the product placed in a common use environment and then the amount of diffusible hydrogen in the steel is measured and found to be 0.20 ppm or less by mass, the amount of diffusible hydrogen in the steel can be regarded as 0.20 ppm or less by mass even before forming or welding.

The microstructure of the steel sheet according to aspects of the present invention will be described below.

Total Area Percentage of One or Two of Martensite Containing Carbide Having Average Particle Size of 50 nm or Less and Bainite Containing Carbide Having Average Particle Size of 50 nm or Less Is 90% or More

To obtain high strength of TS 1,320 MPa, the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more with respect to the entire steel microstructure. At less than this value, ferrite is increased to deteriorate the strength. The total area percentage of the martensite and the bainite may be 100% with respect to the entire steel microstructure. The area percentage of one of the martensite and the bainite may be in the above range, and the total area percentage of both of them may be in the above range. The martensite is defined as the total of as-quenched martensite and tempered martensite. In accordance with aspects of the present invention, martensite refers to a hard microstructure formed from austenite at a low temperature (martensitic transformation temperature or lower). Tempered martensite refers to a microstructure that has been subjected to tempering at the time of reheating martensite. Bainite refers to a hard microstructure in which fine carbides are dispersed in acicular or plate-like ferrite and which is formed from austenite at a relatively low temperature (martensite transformation temperature or higher).

The residual microstructure other than the martensite or the bainite includes, for example, ferrite, pearlite, and retained austenite. When the total amount thereof is, by area percentage, 10% or less, the residual microstructure is allowable. The area percentage of the residual microstructure may be 0%. In accordance with aspects of the present invention, ferrite refers to a microstructure that is formed by transformation from austenite at a relatively high temperature and that is grains with a bcc lattice. Pearlite refers to a layered microstructure composed of layers of ferrite and cementite. Retained austenite refers to austenite that does not transform to martensite when a martensitic transformation temperature is equal to or lower than room temperature. In accordance with aspects of the present invention, the area percentage of each phase in the steel microstructure is determined by a method described in Examples.

Total Area Percentage of One or Two of Martensite Containing Carbide Having Average Particle Size of 50 nm or Less and Bainite Containing Carbide Having Average Particle Size of 50 nm or Less in Region Extending from Surface of Base Steel Sheet to Depth of ⅛ of Thickness of Base Steel Sheet Is 80% or More

Cracking due to bending occurs from a surface layer in a ridge line portion formed by bending of a plated steel sheet; thus, the microstructure of the surface layer portion of the steel sheet is significantly important. In accordance with aspects of the present invention, the use of fine carbides in the surface layer portion as a hydrogen-trapping site reduces the amount of diffusible hydrogen in the vicinity of the surface layer of the steel to improve the bendability. Accordingly, in the case where the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less in a region extending from a surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet is 80% or more, desired bendability can be ensured. The area percentage is preferably 82% or more, more preferably 85% or more. The upper limit of the area percentage is not particularly limited and may be 100%. In the region described above, one of the martensite and the bainite may be in the above range, and the total area percentage of both of them may be in the above range.

Total Perimeter of Individual Carbide Particles Having Average Particle Size of 50 nm or Less in Martensite Containing Carbide Having Average Particle Size of 50 nm or Less and Bainite Containing Carbide Having Average Particle Size of 50 nm or Less Present in Region Extending from Surface of Base Steel Sheet to Depth of ⅛ of Thickness of Base Steel Sheet Is 50 μm/mm2 or More

The amount of diffusible hydrogen in the surface layer portion of the steel is reduced by an increase in the surface area of fine carbide particles present in the vicinity of the surface layer. Thus, the increase in the surface area of fine carbide particles is important. In accordance with aspects of the present invention, as an index of the surface area of fine carbide particles, perimeters of fine carbide particles are used. The total perimeter of carbide particles having an average particle size of 50 nm or less in martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less present in a region extending from a surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet is 50 μm/mm2 or more (50 μm or more per 1 mm2). The total perimeter of the carbide particles is preferably 55 μm/mm2 or more, more preferably 60 μm/mm2 or more. In accordance with aspects of the present invention, the total perimeter of the carbide particles is determined by a method described in Examples.

The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention includes an electrolytic zinc-based coating on a surface of a steel sheet serving as a base (base steel sheet). The type of the zinc-based coating is not particularly limited and may be, for example, a zinc coating (pure Zn) or a zinc alloy coating (e.g., Zn—Ni, Zn—Fe, Zn—Mn, Zn—Cr, or Zn—Co). The coating weight of the electrolytic zinc-based coating is preferably 25 g/m2 or more per one surface from the viewpoint of improving corrosion resistance. The coating weight of the electrolytic zinc-based coating is preferably 50 g/m2 or less per one surface from the viewpoint of not deteriorating the bendability. The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention may include the electrolytic zinc-based coating on one surface of the base steel sheet or may include the electrolytic zinc-based coating on each surface of the base steel sheet. The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention preferably includes the electrolytic zinc-based coating on each surface of the base steel sheet when used for automobiles.

The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention has a tensile strength of 1,320 MPa or more. The tensile strength is preferably 1,400 MPa or more, more preferably 1,470 MPa or more, even more preferably 1,600 MPa or more. The upper limit of the tensile strength is preferably, but not necessarily, 2,200 MPa or less from the viewpoint of easily achieving a balance with other characteristics.

The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention has an elongation (El) of 7.0% or more. The elongation is preferably 7.2% or more, more preferably 7.5% or more. Additionally, TS (MPa)×El (%) is 12,000 or more. TS×El is preferably 13,000 or more, more preferably 13,500 or more. Each of the tensile strength (TS) and the elongation (El) is measured by a method described in Examples.

The limit bending radius/thickness (R/t) of the high-ductility, high-strength electrolytic zinc-based coated steel sheet according to aspects of the present invention is 4.0 or less in a predetermined bending test (bending test described in Examples). R/t is preferably 3.8 or less, more preferably 3.6 or less.

A method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to an embodiment of the present invention will be described below.

The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to an embodiment of the present invention includes at least a hot-rolling step, an annealing step, and a coating treatment step. Additionally, a cold-rolling step may be included between the hot-rolling step and the annealing step. A tempering step may be included after the coating treatment step. These steps will be described below. A temperature described below refers to the surface temperature of a slab, a steel sheet, or the like.

(Hot-Rolling Step)

Slab Heating Temperature

A steel slab having the component composition described above is subjected to hot rolling. The use of a slab heating temperature of 1,200° C. or higher promotes the dissolution of sulfide and reduces the segregation of Mn to reduce the amounts of coarse inclusions described above, thereby improving the bendability. For this reason, the slab heating temperature is 1,200° C. or higher. The slab heating temperature is more preferably 1,230° C. or higher, even more preferably 1,250° C. or higher. For example, the heating rate during heating of the slab may be 5 to 15° C./min, and the slab soaking time may be 30 to 100 minutes.

Finish Hot-Rolling Temperature

The finish hot-rolling temperature needs to be 840° C. or higher. At a finish hot-rolling temperature of lower than 840° C., it takes time to reduce the temperature. This may form inclusions to deteriorate the bendability and deteriorate the quality of the inside of the steel sheet. Additionally, decarburization at a surface layer decreases the area percentages of bainite and martensite containing carbides in the surface layer portion of the steel to decrease fine carbides serving as hydrogen-trapping sites in the vicinity of the surface layer, thereby making it difficult to ensure desired bendability. Accordingly, the finish hot-rolling temperature needs to be 840° C. or higher. The finish hot-rolling temperature is preferably 860° C. or higher. The upper limit of the finish hot-rolling temperature is preferably, but not necessarily, 950° C. or lower because a difficulty lies in cooling to a coiling temperature described below. The finish hot-rolling temperature is more preferably 920° C. or lower.

After the completion of the finish hot rolling, cooling is performed to a primary cooling stop temperature of 700° C. or lower at an average cooling rate of 40° C./s or more in a temperature range of the finish hot-rolling temperature to 700° C. A low cooling rate results in the formation of inclusions. An increase in the size of the inclusions deteriorates the bendability. Decarburization at the surface layer decreases area percentages of martensite and bainite containing carbides in the surface layer portion of the steel to decrease fine carbides serving as hydrogen-trapping sites in the vicinity of the surface layer, thereby making it difficult to ensure desired bendability. Accordingly, after the completion of the finish hot rolling, the average cooling rate is 40° C./s or more from the finish hot-rolling temperature to 700° C. The average cooling rate is preferably 50° C./s or more. The upper limit of the average cooling rate is preferably, but not necessarily, about 250° C./s. The primary cooling stop temperature is 700° C. or lower. At a primary cooling stop temperature of higher than 700° C., carbides are easily formed down to 700° C. The coarsening of the carbides deteriorates the bendability. The lower limit of the primary cooling stop temperature is not particularly limited. At a primary cooling stop temperature of 650° C. or lower, the effect of rapid cooling on the suppression of carbide formation is decreased. Thus, the primary cooling stop temperature is preferably higher than 650° C.

After that, cooling is performed at an average cooling rate of 2° C./s or more in a temperature range of the primary cooling stop temperature to 650° C., and then cooling is performed to a coiling temperature of 630° C. or lower. A low cooling rate to 650° C. results in the formation of inclusions. An increase in the size of the inclusions deteriorates the bendability. Decarburization at the surface layer decreases area percentages of martensite and bainite containing carbides in the surface layer portion of the steel to decrease fine carbides serving as hydrogen-trapping sites in the vicinity of the surface layer, thereby making it difficult to ensure desired bendability. Accordingly, as described above, after cooling is performed to a primary cooling stop temperature of 700° C. or lower at an average cooling rate of 40° C./s or more in the temperature range down to 700° C., the average cooling rate is 2° C./s or more in the temperature range of the primary cooling stop temperature to 650° C. The average cooling rate is preferably 3° C./s or more, more preferably 5° C./s. The average cooling rate from 650° C. to the coiling temperature is preferably, but not necessarily, 0.1° C./s or more and 100° C./s or less.

The coiling temperature is 630° C. or lower. A coiling temperature of higher than 630° C. may result in decarburization at the surface of base steel to lead to a difference in microstructure between the inside and the surface of the steel sheet, causing a nonuniformity in alloy concentration. Additionally, decarburization at the surface layer decreases area percentages of martensite and bainite containing carbides in the surface layer portion of the steel to decrease fine carbides serving as hydrogen-trapping sites in the vicinity of the surface layer, thereby making it difficult to ensure desired bendability. Accordingly, the coiling temperature is 630° C. or lower. The coiling temperature is preferably 600° C. or lower. The lower limit of the coiling temperature is not particularly limited. To prevent a decrease in cold rollability when cold rolling is performed, the coiling temperature is preferably 500° C. or higher.

Cold-Rolling Step

After the hot-rolling step, a cold-rolling step may be performed. In the case where the cold-rolling step is performed, in the cold-rolling step, the steel sheet (hot-rolled steel sheet) coiled in the hot-rolled step is subjected to pickling and then cold rolling to produce a cold-rolled steel sheet. The conditions of the pickling are not particularly limited. The rolling reduction is not particularly limited. At a rolling reduction of less than 20%, the surfaces may have poor flatness to lead to a nonuniform microstructure. Thus, the rolling reduction is preferably 20% or more. The cold-rolling step may be omitted as long as the microstructure and the mechanical properties satisfy the requirements of the present invention.

(Annealing Step)

The steel sheet that has been subjected to the hot-rolling step or the cold-rolling step subsequent to the hot-rolling step is heated to an annealing temperature equal to or higher than an AC3 point. An annealing temperature of lower than the AC3 point results in the formation of ferrite in the microstructure to fail to obtain desired strength. Accordingly, the annealing temperature is the AC3 point or higher. The annealing temperature is preferably the AC3 point+10° C. or higher, more preferably the AC3 point+20° C. or higher. The upper limit of the annealing temperature is not particularly limited. From the viewpoint of suppressing the coarsening of austenite to prevent the deterioration of the bendability, the annealing temperature is preferably 900° C. or lower. The atmosphere during annealing is not particularly limited. From the viewpoint of preventing decarburization in the surface layer portion, the dew point is preferably −50° C. or higher and −5° C. or lower.

The AC3 point (° C.) used here is calculated from the following formula. In the formula, each (% symbol of element) refers to the amount of the corresponding element contained (% by mass).


AC3 point=910-203(% C)1/2+45(% Si)-30(% Mn)-20(% Cu)-15(% Ni)+11(% Cr)+32(% Mo)+104(% V)+400(% Ti)+460(% Al)

After heating is performed to the annealing temperature equal to or higher than the AC3 point, cooling is performed to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./s or more in a temperature range of the annealing temperature to 550° C., and holding is performed at a holding temperature in a temperature range of 100° C. to 200° C. for 20 to 1,500 seconds. After heating to the annealing temperature equal to or higher than the AC3 point, soaking may be performed at the annealing temperature. The soaking time here is preferably, but not necessarily, 10 seconds or more and 300 seconds or less, more preferably 15 seconds or more and 250 seconds or less. An average cooling rate of less than 3° C./s in the temperature range of the annealing temperature to 550° C. leads to excessive formation of ferrite to make it difficult to obtain desired strength. Additionally, the formation of ferrite in the surface layer portion makes it difficult to increase the fractions of the martensite and bainite containing carbides in the vicinity of the surface layer, thereby deteriorating the bendability. Accordingly, the average cooling rate in the temperature range of the annealing temperature to 550° C. is 3° C./s or more, preferably 5° C./s or more, more preferably 10° C./s or more.

The cooling stop temperature is 350° C. or lower. A cooling stop temperature of higher than 350° C. results in the formation of bainite containing coarse carbides to decrease the amount of fine carbides in the surface layer portion of the steel, thereby deteriorating the bendability.

The average cooling rate is defined by (the cooling start temperature−the cooling stop temperature)/the cooling time from the cooling start temperature to the cooling stop temperature, unless otherwise specified.

Then holding is performed at a holding temperature in the temperature range of 100° C. to 200° C. for 20 to 1,500 seconds. The carbides distributed in the bainite are carbides formed during the holding in the low temperature range after quenching and serve as hydrogen-trapping sites to trap hydrogen, and can prevent the deterioration of the bendability. When the holding temperature is lower than 100° C. or when the holding time is less than 20 seconds, bainite is not formed, and as-quenched martensite containing no carbide is formed. Thus, the amount of fine carbides in the surface layer portion of the steel is decreased to fail to provide the above effect. When the holding temperature is higher than 200° C. or when the holding time is more than 1,500 seconds, decarburization occurs, and coarse carbides are formed in the bainite, thereby deteriorating the bendability. The holding temperature is preferably 120° C. or higher. The holding temperature is preferably 180° C. or lower. The holding time is preferably 50 seconds or more. The holding time is preferably 1,000 seconds or less.

After the annealing step, cooling is performed to room temperature. The cooling rate at this time is not particularly limited. Down to 50° C., the average cooling rate is preferably 1° C./s or more. The term “room temperature” indicates, for example, 10° C. to 30° C.

(Coating Treatment Step)

After cooling to room temperature, the steel sheet is subjected to electrolytic zinc-based coating. The type of the electrolytic zinc-based coating may be, but is not particularly limited to, any of pure Zn, Zn—Ni, Zn—Fe, Zn—Mn, Zn—Cr, Zn—Co, and so forth. To suppress the entry of hydrogen into the steel and to achieve the amount of diffusible hydrogen in the steel of the electrolytic zinc-based coated steel sheet to 0.20 ppm or less by mass, the electroplating time is important. At an electroplating time of more than 300 seconds, the steel sheet is immersed in an acid for a long time; thus, the amount of diffusible hydrogen in the steel is more than 0.20 ppm by mass, thereby deteriorating the bendability. Accordingly, the electroplating time is 300 seconds or less. The electroplating time is preferably 280 seconds or less, more preferably 250 seconds or less.

The steel sheet after the coating treatment step (electrolytic zinc-based coated steel sheet) may be subjected to the tempering step. The amount of diffusible hydrogen in the steel can be reduced through the tempering step to further enhance the bendability. The tempering step is preferably a step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:


(T+273)(log t+4)≤2,700  (1)

where in formula (1), T is the holding temperature (° C.) in the tempering step, and t is the holding time (seconds) in the tempering step.

In the production method according to the embodiment described above, the high-ductility, high-strength electrolytic zinc-based coated steel sheet having excellent bendability can be produced by controlling the production condition of the base steel sheet before the coating treatment step and the coating treatment conditions so as to form fine carbides in the surface layer portion of the steel and use the fine carbides as hydrogen-trapping sites to reduce the amount of diffusible hydrogen in the steel.

The hot-rolled steel sheet after the hot-rolling step may be subjected to heat treatment for softening the microstructure. After the coating treatment step, temper rolling may be performed for shape adjustment.

EXAMPLES

The present invention will be specifically described below with reference to Examples.

1. Production of Steel Sheet for Evaluation

Molten steels having component compositions given in Table 1, the balance being Fe and incidental impurities, were produced with a vacuum melting furnace. Each steel was subjected to blooming into a steel slab having a thickness of 27 mm. The resulting steel slab was hot-rolled into a hot-rolled steel sheet having a thickness of 4.0 mm (hot-rolling step). Regarding samples to be subjected to cold rolling, the hot-rolled steel sheets were processed by grinding into a thickness of 3.2 mm and then cold-rolled at rolling reductions given in Tables 2-1 to 2-4 into cold-rolled steel sheets having a thickness of 1.4 mm (cold-rolling step). In Table 2-1, samples in which numerical values of the rolling reduction in the cold rolling are not described were not subjected to cold rolling. The hot-rolled steel sheets and the cold-rolled steel sheets produced as described above were subjected to heat treatment (annealing step) and coating (coating treatment step) under conditions given in Tables 2-1 to 2-4 to produce electrolytic zinc-based coated steel sheets. Blanks in Table 1 presenting the component composition indicate that the components are intentionally not added, and the blanks also include the case where the components are not contained (0% by mass) and the case where the components are incidentally contained. Some samples were subjected to the tempering step. In Tables 2-1 to 2-4, tempering condition cells that are blank indicate that no tempering step was performed.

In the coating treatment step, in the case of pure Zn coating, an electroplating solution prepared by adding 440 g/L of zinc sulfate heptahydrate to deionized water and adjusting the pH to 2.0 with sulfuric acid was used. For Zn—Ni coating, an electroplating solution prepared by adding 150 g/L of zinc sulfate heptahydrate and 350 g/L of nickel sulfate hexahydrate to deionized water and adjusting the pH to 1.3 with sulfuric acid was used. In the case of Zn—Fe coating, an electroplating solution prepared by adding 50 g/L of zinc sulfate heptahydrate and 350 g/L of iron sulfate to deionized water and adjusting the pH to 2.0 with sulfuric acid was used. Inductively coupled plasma (ICP) analysis of the coatings revealed that the alloy compositions of the coatings were 100% Zn, Zn-13% Ni, and Zn-46% Fe. The coating weight of each electrolytic zinc-based coating was 25 to 50 g/m2 per one surface. Specifically, the coating composed of 100%-Zn had a coating weight of 33 g/m2 per one surface. The coating composed of Zn-13% Ni had a coating weight of 27 g/m2 per one surface. The coating composed of Zn-46% Fe had a coating weight of 27 g/m2 per one surface. These electrolytic zinc-based coatings were formed on both surfaces of the steel sheets.

TABLE 1 Steel Component composition (% by mass) grade C Si Mn P S Al N Sb B Nb Ti Cu Ni A 0.24 1.0 3.0 0.007 0.0008 0.051 0.0021 0.01 B 0.17 1.1 2.9 0.008 0.0003 0.068 0.0048 0.01 C 0.13 1.0 3.0 0.008 0.0005 0.080 0.0021 0.02 D 0.27 1.2 2.9 0.018 0.0002 0.021 0.0043 0.01 E 0.35 1.2 3.0 0.010 0.0010 0.077 0.0043 0.01 F 0.24   0.002 3.5 0.010 0.0010 0.049 0.0058 0.04 G 0.22 1.7 3.4 0.007 0.0004 0.036 0.0014 0.01 H 0.22 0.9 1.8 0.007 0.0010 0.078 0.0034 0.02 I 0.21 0.8 2.5 0.006 0.0007 0.096 0.0046 0.03 J 0.23 0.9 4.9 0.025 0.0002 0.092 0.0028 0.01 K 0.19 1.0 3.5 0.009 0.0009 0.026 0.0031  0.005 L 0.22 0.9 3.7 0.016 0.0004 0.039 0.0028  0.003 M 0.23 0.8 3.4 0.005 0.0004 0.050 0.0015 0.07 N 0.22 0.8 3.5 0.006 0.0010 0.066 0.0053 0.09 O 0.23 1.1 3.6 0.038 0.0006 0.051 0.0040 0.01 P 0.19 0.1 2.9 0.006 0.0002 0.062 0.0027 0.01 0.0020 Q 0.24 0.8 3.1 0.009 0.0002 0.063 0.0051 0.05 0.0200 R 0.20 0.1 3.1 0.007 0.0004 0.038 0.0051 0.01 0.017 S 0.25 0.1 2.8 0.006 0.0003 0.040 0.0037 0.01 0.0015 0.0150 0.015 T 0.18 0.8 3.4 0.017 0.0005 0.034 0.0019 0.06 0.12 U 0.21 0.2 3.5 0.009 0.0003 0.096 0.0060 0.01 0.15 0.04 V 0.20 0.6 3.4 0.025 0.0010 0.096 0.0020 0.02 W 0.24 0.1 4.1 0.008 0.0010 0.068 0.0020 0.02 X 0.22 0.4 4.0 0.009 0.0001 0.057 0.0043 0.01 Y 0.23 1.1 3.5 0.009 0.0009 0.042 0.0029 0.03 Z 0.20 1.0 3.4 0.009 0.0007 0.034 0.0039 0.03 AA 0.18 0.8 3.4 0.045 0.0010 0.034 0.0033 0.04 0.0015 0.0150 0.01 AB 0.22 0.4 3.2 0.007 0.0007 0.060 0.0027 0.01 AC 0.42 1.1 3.2 0.019 0.0002 0.035 0.0021 0.01 AD 0.08 1.0 3.0 0.006 0.0002 0.077 0.0055 0.01 AE 0.21 2.4 3.1 0.008 0.0010 0.023 0.0028 0.01 AF 0.22 1.1 1.5 0.026 0.0006 0.069 0.0024 0.01 AG 0.21 0.8 3.1 0.070 0.0007 0.059 0.0010 0.01 AH 0.19 0.8 3.2 0.018 0.0080 0.069 0.0058 0.01 AI 0.22 1.1 2.8 0.007 0.0004 0.250 0.0028 0.01 AJ 0.25 0.8 3.3 0.006 0.0003 0.064 0.0150 0.01 AK 0.21 0.6 3.3 0.018 0.0008 0.071 0.0017  0.001 AL 0.18  0.01 3.1 0.009 0.0005 0.076 0.0015 0.15 Steel Component composition (% by mass) AC3 grade Cr Mo V Zr W Ca Ce La Mg Sn point A 789 B 820 C 829 D 781 E 789 F 728 G 806 H 837 I 822 J 748 K 773 L 763 M 770 N 776 O 778 P 767 Q 780 R 755 S 753 T 771 U 762 V 0.15 790 W 0.2  731 X 0.17 0.15 0.02 746 Y 0.012 0.01 0.0008 0.0009 0.0006 0.0004 776 Z 0.004 778 AA 0.01  778 AB 764 AC 748 AD 843 AE 843 AF 851 AG 787 AH 795 AI 895 AJ 775 AK 778 AL 767 Underlined values are outside the scope of the present invention.

TABLE 2-1 Hot rolling Finish Average Average Cold Slab hot- cooling cooling rolling Annealing heating rolling rate rate Coiling Rolling Annealing Average temper- temper- to to temper- re- temper- Dew cooling Steel ature ature 700° C. *1 650° C. *2 ature duction ature point rate No. grade ° C. ° C. ° C./s ° C./s ° C. % ° C. ° C. ° C./S *3  1 A 1250 880 232 31 550 56 820 −15 28  2 1250 880 245 33 550 56 825 −15 26  3 1250 880 225 32 550 56 830 −15 27  4 1250 880 246 34 550 56 830 −15 30  5 1250 880 248 50 550 56 840 −15 25  6 1250 880 247 18 550 56 840 −15 34  7 1250 880 239 13 550 56 860 −15 25  8 1250 880 251  1 550 56 830 −15 27  9 B 1250 880 235 33 550 56 887 −15 30 10 1240 880 237 35 550 56 902 −15 24 11 1210 880 241 37 550 56 896 −15 25 12 1180 880 242 34 550 56 890 −15 29 13 C 1250 900 239 38 550 56 863 −15 35 14 1250 880 242 35 550 56 904 −15 28 15 1250 850 250 36 550 56 894  −5 27 16 1250 820 247 34 550 56 862 −15 26 17 D 1250 880 250 32 550 56 822 −15 30 18 1250 880 100 31 550 56 830 −15 25 19 1250 880  40 38 550 56 834  −6 28 20 1250 880  20 34 550 56 848 −15 30 21 E 1250 880 228 30 550 56 817 −15 26 22 1250 880 229 35 580 56 833 −15 37 23 1250 880 231 37 620 56 849 −15 30 24 1250 880 234 34 650 56 840 −15 26 25 F 1250 880 227 35 550 — 804 −15 25 26 1250 880 229 33 550 — 812 −15 28 27 1250 880 230 32 550 — 830 −15 30 28 1250 880 231 36 550 785 −15 34 29 G 1250 880 230 35 550 56 846 −15 28 30 1250 880 234 38 550 56 835 −15 27 31 1250 880 238 37 550 56 830 −15 30 32 1250 880 237 34 550 56 800 −15 26 Tempering Annealing condition Cooling Hol- stop Holding Hol- Coating ding Hol- temper- temper- ding Type Plating temper- ding Steel ature ature time of time ature time No. grade ° C. ° C. s coating s ° C. s  1 A 150 150 150 Zn 120 Example  2 150 150 150 Zn 180 250  10 Example  3 150 150 150 Zn 260  80 3600 Example  4 150 170 150 Zn 320 Com- parative example  5 150 170 150 Zn 230 Example  6 150 170 150 Zn 230 Example  7 150 170 150 Zn 230 Example  8 150 170 150 Zn 240 Com- parative example  9 B 150 170 150 Zn 230 Example 10 150 170 150 Zn 230 Example 11 150 170 150 Zn 250 Example 12 150 170 150 Zn 230 Com- parative example 13 C 150 170 150 Zn 260 Example 14 150 170 150 Zn 230 Example 15 150 170 150 Zn 230 Example 16 150 170 150 Zn 230 Com- parative example 17 D 150 170 150 Zn 240 200   30 Example 18 150 170 150 Zn 230 150  180 Example 19 150 170 150 Zn 250 Example 20 150 170 150 Zn 230 Com- parative example 21 E 150 170 150 Zn 260 Example 22 150 170 150 Zn 230 Example 23 150 170 150 Zn 230 Example 24 150 170 150 Zn 230 Com- parative example 25 F 150 170 150 Zn 260 Example 26 150 170 150 Zn 230 Example 27 150 170 150 Zn 250 Example 28 150 170 150 Zn 240 Example 29 150 170 150 Zn 230 Example 30 150 170 150 Zn 260 Example 31 150 170 150 Zn 230 Example 32 150 170 150 Zn 250 Com- parative Example *1 The average cooling rate from the finish hot-rolling temperature to 700° C. *2 The average cooling rate from 700° C. (primary cooling stop temperature) to 650° C. *3 The average cooling rate in the temperature range of the annealing temperature to 550° C. Underlined values are outside the scope of the present invention.

TABLE 2-2 Hot rolling Finish Average Average Cold Slab hot- cooling cooling rolling Annealing heating rolling rate rate Coiling Rolling Annealing Average temper- temper- to to temper- re- temper- Dew cooling Steel ature ature 650° C. *1 700° C. *2 ature duction ature point rate No. grade ° C. ° C. ° C./s ° C./s ° C. % ° C. ° C. ° C./S *3 33 H 1250 880 241 31 550 56 865 −15 30 34 1250 880 235 32 550 56 870 −15 18 35 1250 880 236 33 550 56 880 −19  6 36 1250 880 238 35 550 56 870 −15  2 37 I 1250 880 244 36 550 56 850 −15 28 38 1250 880 241 38 550 56 860 −15 27 39 1250 880 237 39 550 56 854 −15 26 40 1250 880 229 34 550 56 880 −15 30 41 J 1250 880 235 35 550 56 790 −15 25 42 1250 880 234 31 550 56 780 −15 35 43 1250 880 228 30 550 56 820 −15 29 44 1250 880 229 32 550 56 819 −15 30 45 K 1250 880 230 35 550 56 809 −15 27 46 1250 880 247 37 550 56 816 −15 28 47 1250 880 246 36 550 56 804  −5 27 48 1250 880 241 34 550 56 820 −15 30 49 L 1250 880 300 33 550 56 793 −15 26 50 1250 880 220 32 550 56 801 −15 35 51 1250 880 150 35 550 56 821  −7 29 52 1250 880  15 38 550 56 810 −15 27 53 M 1250 880 247 30 550 56 801 −15 28 54 1250 880 242 21 550 56 795 −15 29 55 1250 880 245 14 550 56 823 −15 30 56 1250 880 239  1 550 56 818 −15 38 57 N 1250 880 234 34 550 56 806 −15 27 58 1250 880 235 35 550 56 815 −15 29 59 1250 880 237 36 550 56 831 −15 28 60 1250 880 236 32 550 56 824 −15 28 Tempering Annealing condition Cooling Hol- stop Holding Hol- Coating ding Hol- temper- temper- ding Type Plating temper- ding Steel ature ature time of time ature time No. grade ° C. ° C. s coating s ° C. s 33 H 150 170 150 Zn 230 Example 34 150 170 150 Zn 230 Example 35 150 170 150 Zn 260 Example 36 150 170 150 Zn 230 Com- parative example 37 I 370 170 150 Zn 240 Com- parative example 38 340 170 150 Zn 230 Example 39 320 170 150 Zn 230 Example 40 120 170 150 Zn 250 Example 41 J 150 170 1750  Zn 230 Com- parative example 42 150 170 800 Zn 260 Example 43 150 170 100 Zn 230 Example 44 150 170   8 Zn 230 Com- parative example 45 K 150  90 150 Zn 200 Com- parative example 46 150 150 150 Zn 180 Example 47 150 170 150 Zn 160 Example 48 150 220 150 Zn 120 Com- parative example 49 L 150 170 150 Zn 230 Example 50 150 170 150 Zn 230 Example 51 150 170 150 Zn 230 Example 52 150 170 150 Zn 240 Com- parative example 53 M 150 170 150 Zn 230 Example 54 150 170 150 Zn 230 Example 55 150 170 150 Zn 250 Example 56 150 170 150 Zn 230 Com- parative example 57 N 150 170 150 Zn—Ni 400 Com- parative example 58 150 170 150 Zn—Ni 310 Com- parative example 59 150 170 150 Zn—Ni 240 Example 60 150 170 150 Zn—Ni 130 Example *1 The average cooling rate from the finish hot-rolling temperature to 700° C. *2 The average cooling rate from 700° C. (primary cooling stop temperature) to 650° C. *3 The average cooling rate in the temperature range of the annealing temperature to 550° C. Underlined values are outside the scope of the present invention.

TABLE 2-3 Hot rolling Finish Average Average Cold Slab hot- cooling cooling rolling Annealing heating rolling rate rate Coiling Rolling Annealing Average temper- temper- to to temper- re- temper- Dew cooling Steel ature ature 700° C. *1 650° C. *2 ature duction ature point rate No. grade ° C. ° C. ° C./s ° C./s ° C. % ° C. ° C. ° C./S *3 61 O 1250 880 180 31 550 56 811 −15 29 62 1250 880 120 30 550 56 807 −27 30 63 1250 880  60 37 550 56 830 −15 29 64 1250 880  35 35 550 56 806 −15 27 65 P 1250 880 237 38 550 56 793 −15 28 66 1250 880 235 34 550 56 807 −15 36 67 1250 880 233 35 550 56 820 −15 27 68 1250 880 238 31 550 56 814  −7 30 69 Q 1250 880 241 32 550 56 802 −30 29 70 1250 880 240 35 550 56 811 −15 28 71 1250 880 241 33 550 56 834 −15 29 72 1250 880 240 34 550 56 822 −35 37 73 R 1250 880 246 36 550 56 789 −15 30 74 1250 880 238 31 550 56 781 −15 29 75 1250 880 237 32 550 56 805  15 28 76 1250 880 237 34 550 56 810  −6 26 77 S 1250 880 235 37 550 56 787 −15 28 78 1250 880 239 38 550 56 798 −15 27 79 1250 880 242 35 550 56 810 −15 30 80 1250 880 243 39 550 56 794 −15 29 81 T 1250 880 400 35 550 56 808 −15 33 82 1250 880 140 34 550 56 819 −15 27 83 1250 880  30 32 550 56 824 −15 28 85 U 1250 880 1148  36 550 56 798 −15 30 86 1250 880 500 32 550 56 789 −15 28 87 1250 880 170 31 550 56 808 −26 29 88 1250 880  35 30 550 56 804 −15 27 89 V 1250 880 110 35 550 56 816 −15 28 90 1250 880  70 37 550 56 827 −15 26 91 1250 880  30 38 550 56 830 −15 29 92 1250 880 1187  36 550 56 824 −15 30 Tempering Annealing condition Cooling Hol- stop Holding Hol- Coating ding Hol- temper- temper- ding Type Plating temper- ding Steel ature ature time of time ature time No. grade ° C. ° C. s coating s ° C. s 61 O 150 170 150 Zn—Ni 230 Example 62 150 170 150 Zn—Ni 230 Example 63 150 170 150 Zn—Ni 260 Example 64 150 170 150 Zn—Ni 240 Com- parative example 65 P 150 150  80 Zn—Ni 230 Example 66 150 150 1840  Zn—Ni 230 Com- parative example 67 150 150   8 Zn—Ni 260 Com- parative example 68 150 150 600 Zn—Ni 230 Example 69 Q 150 150 300 Zn—Ni 250 Example 70 150 150 1630  Zn—Ni 230 Com- parative example 71 150 150   7 Zn—Ni 230 Com- parative example 72 150 150  60 Zn—Ni 240 Example 73 R 150 150 1720  Zn—Ni 230 Com- parative example 74 150 150   6 Zn—Ni 230 Com- parative example 75 150 150 1200  Zn—Ni 260 Example 76 150 150 900 Zn—Ni 250 Example 77 S 150 150 1750  Zn—Fe 230 Com- parative example 78 150 150 500 Zn—Fe 230 Example 79 150 230 200 Zn—Fe 230 Com- parative example 80 150  80 400 Zn—Fe 240 Com- parative example 81 T 150 150 150 Zn—Fe 230 Example 82 150 150 150 Zn—Fe 230 Example 83 150 150 150 Zn—Fe 260 Com- parative example 85 U 150 150 150 Zn—Fe 230 100 120 Example 86 150 150 150 Zn—Fe 250 Example 87 150 150 150 Zn—Fe 230 Example 88 150 150 150 Zn—Fe 230 Com- parative example 89 V 150 150 150 Zn Fe 260 Example 90 150 150 150 Zn—Fe 230 Example 91 150 150 150 Zn—Fe 240 Com- parative example 92 150 150 150 Zn—Fe 230 Example *1 The average cooling rate from the finish hot-rolling temperature to 700° C. *2 The average cooling rate from 700° C. (primary cooling stop temperature) to 650° C. *3 The average cooling rate in the temperature range of the annealing temperature to 550° C. Underlined values are outside the scope of the present invention.

Underlined values are outside the scope of the present invention.

TABLE 2-4 Hot rolling Finish Average Average Cold Slab hot- cooling cooling rolling Annealing heating rolling rate rate Coiling Rolling Annealing Average temper- temper- to to temper- re- temper- Dew cooling Steel ature ature 700° C. *1 650° C. *2 ature duction ature point rate No. grade ° C. ° C. ° C./s ° C./s ° C. % ° C. ° C. ° C./S *3  93 W 1250 880 130 35 550 56 760 −15 28  94 1250 880  60 38 550 56 779 −15 32  95 1250 880  15 35 550 56 790 −15 29  96 1250 880 120 34 550 56 783 −15 28  97 X 1250 880 238 1124   550 56 776 −15 29  98 1250 880 237 160  550 56 798 −15 27  99 1250 880 234  1 550 56 805 −15 28 100 1250 880 241 48 550 56 788 −15 28 101 Y 1250 880 246 71 550 56 808 −15 30 102 1250 880 242  1 550 56 804 −15 29 103 1250 880 236 34 550 56 806 −27 27 104 1250 880 235 41 550 56 813  −5 26 105 Z 1250 880 233 75 550 56 804 −15 34 106 1250 880 232 90 550 56 814 −15 27 107 1250 880 228 840  550 56 823 −15 30 108 1250 880 229  1 550 56 805 −15 28 109 AA 1250 880 227 34 550 56 808  −5 29 110 1250 880 230 32 550 56 812 −15 31 111 1250 880 229 31 550 56 825 −15 27 112 1250 880 225 30 550 56 806 −15 27 113 AB 1250 880 234 35 550 56 790 −15 30 114 1250 880 236 38 550 56 793 −15 29 115 1250 880 228 37 550 56 809 −30 28 116 1250 880 229 35 550 56 795 −15 33 117 AC 1250 880 230 36 550 56 783 −15 29 118 AD 1250 880 240 35 550 56 874 −15 27 119 AE 1250 880 231 34 550 56 882 −15 30 120 AF 1250 880 242 36 550 56 884 −15 28 121 AG 1250 880 250 33 550 56 820 −15 29 122 AH 1250 880 237 32 550 56 830 −15 30 123 AI 1250 880 240 35 550 56 929 −15 28 124 AJ 1250 880 245 35 550 56 802 −15 27 125 AK 1250 880 237 36 550 56 816 −15 26 126 AL 1250 880 239 30 550 56 807 −15 30 Tempering Annealing condition Cooling Hol- stop Holding Hol- Coating ding Hol- temper- temper- ding Type Plating temper- ding Steel ature ature time of time ature time No. grade ° C. ° C. s coating s ° C. s  93 W 150 150 150 Zn—Fe 250 150  20 Example  94 150 150 150 Zn—Fe 230 150 150 Example  95 150 150 150 Zn—Fe 230 Com- parative example  96 150 150 150 Zn—Fe 230 Example  97 X 150 150 150 Zn—Ni 230 Example  98 150 150 150 Zn—Ni 250 Example  99 150 150 150 Zn—Ni 230 Com- parative example 100 150 150 150 Zn—Ni 230 Example 101 Y 150 150 150 Zn—Ni 240 Example 102 150 150 150 Zn—Ni 230 Com- parative example 103 150 150 150 Zn—Ni 260 Example 104 150 150 150 Zn—Ni 230 Example 105 Z 150 150 150 Zn—Ni 250 Example 106 150 150 150 Zn—Ni 230 Example 107 150 150 1200 Zn—Ni 230 Example 108 150 150 150 Zn—Ni 260 Com- parative example 109 AA 150 150 150 Zn—Ni 230 Example 110 270 120 150 Zn—Ni 240 Example 111 320 120 150 Zn—Ni 230 Example 112 370 200 150 Zn—Ni 250 Com- parative example 113 AB 150 200 150 Zn—Ni 230 Example 114 360 200 150 Zn—Ni 260 Com- parative example 115 300 200 150 Zn—Ni 230 Example 116 150 200 150 Zn—Ni 230 Example 117 AC 150 150 150 Zn—Ni 240 Com- parative example 118 AD 150 150 150 Zn—Ni 230 Com- parative example 119 AE 150 150 150 Zn—Ni 230 Com- parative example 120 AF 150 150 150 Zn—Ni 230 Com- parative example 121 AG 150 150 150 Zn—Ni 230 Com- parative example 122 AH 150 150 150 Zn Ni 230 Com- parative example 123 AI 150 150 150 Zn—Ni 230 Com- parative example 124 AJ 150 150 150 Zn—Ni 230 Com- parative example 125 AK 150 150 150 Zn—Ni 230 Com- parative example 126 AL 150 150 150 Zn—Ni 230 Com- parative example *1 The average cooling rate from the finish hot-rolling temperature to 700° C. *2 The average cooling rate from 700° C. (primary cooling stop temperature) to 650° C. *3 The average cooling rate in the temperature range of the annealing temperature to 550° C. Underlined values are outside the scope of the present invention.

2. Evaluation Method

With respect to the electrolytic zinc-based coated steel sheets produced under various production conditions, the microstructure fractions were examined by the analysis of the steel microstructures. The tensile characteristics, such as tensile strength, were evaluated by conducting a tensile test. The bendability was evaluated by a bending test. Evaluation methods were described below.

(Total Area Percentage of One or Two of Martensite Containing Carbide Having Average Particle Size of 50 nm or Less and Bainite Containing Carbide Having Average Particle Size of 50 nm or Less)

A test piece was taken from a portion of each of the electrolytic zinc-based coated steel sheets in the rolling direction and a direction perpendicular to the rolling direction. An L-cross-section extending in the thickness direction and a direction parallel to the rolling direction was mirror-polished, etched with Nital to reveal microstructures, and observed with a scanning electron microscope. The area percentage of each of martensite and bainite was examined by a point counting method in which a 16×15 grid of points at 4.8 μm intervals was placed on a region, measuring 82 μm×57 μm in terms of actual length, of a SEM image with a magnification of ×1,500 and the points on each phase were counted. The area percentage of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less in the entire microstructure was defined as the average value of their area percentages from SEM images obtained by continuous observation of the entire cross-section in the thickness direction at a magnification of ×1,500. The area percentage of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less in a region extending a surface of a base steel sheet to a depth of ⅛ of the thickness of the base steel sheet was defined as the average value of their area percentages from SEM images obtained by continuous observation of the region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet at a magnification of ×1,500. Martensite and bainite appear as white microstructures in which blocks and packets are revealed within prior austenite grain boundaries and fine carbides are precipitated therein. A difficulty may lie in revealing carbides therein, depending on the crystallographic orientation of a block grain and the degree of etching. In that case, it is necessary to sufficiently perform etching and check it. The average particle size of the carbides in the martensite and the bainite was calculated by a method described below.

(Average Particle Size of Carbide in Martensite and Bainite)

A test piece was taken from a portion of each of the electrolytic zinc-based coated steel sheets in the rolling direction and a direction perpendicular to the rolling direction. An L-cross-section extending in the thickness direction and a direction parallel to the rolling direction was mirror-polished, etched with Nital to reveal microstructures, and observed with a scanning electron microscope. The number of carbides in prior austenite grains containing martensite and bainite was calculated from one SEM image obtained by continuous observation of the region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet at a magnification of ×5,000. The total area of carbides in one grain was calculated by binarization of the microstructure. The area of one carbide particle was calculated from the number and the total area of the carbides. The average particle size of the carbides in the region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet was calculated. A method for measuring the average particle size of the carbides in the entire microstructure is as follows: A point located at a depth of ¼ of the thickness of the base steel sheet was observed with a scanning electron microscope. Then the average particle size of the carbides in the entire microstructure was measured in the same way as the method for calculating the average particle size of the carbides in the region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet. Here, the microstructure located at a depth of ¼ of the thickness of the base steel sheet was regarded as the average microstructure of the entire microstructure.

(Total Perimeter of Carbide Particles Having Average Particle Size of 50 nm or Less)

The total perimeter of individual carbide particles having an average particle size of 50 nm or less in martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less present in the region extending from the surface of the base steel sheet to a depth of ⅛ of the thickness of the base steel sheet was determined as follows: Regarding the individual carbide particles having an average particle size of 50 nm or less in martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less present in the region, the perimeters of the individual carbide particles were calculated by multiplying the average particle size of the individual carbide particles by circular constant pi 7c. The average of the resulting perimeters was determined. The total perimeter was determined by multiplying the average by the number of the carbide particles having an average particle size of 50 nm or less. The average particle size of the individual carbide particles is defined as the average value of lengths of the long axes and the short axes of the images of the carbide particles when the microstructure was binarized as described above.

(Tensile Test)

JIS No. 5 test pieces having a gauge length of 50 mm, a gauge width of 25 mm, and a thickness of 1.4 mm were taken from the electrolytic zinc-based coated steel sheets in the rolling direction and subjected to a tensile test at a cross head speed of 10 mm/min to measure tensile strength (TS) and elongation (El).

(Bending Test)

Bending test pieces having a width of 25 mm and a length of 100 mm were taken from the electrolytic zinc-based coated steel sheets in such a manner that the rolling direction was a bending direction. The test pieces were subjected to a test (n=3) by a pressing bend method according to JIS Z 2248 at a pressing rate of 100 mm/s and various bending radii. A bending radius at which no crack was formed in three test pieces was defined as a limit bending radius. Evaluation was performed on the basis of the ratio of the limit bending radius to the thickness of the steel sheet. Here, the presence or absence of a crack was checked by observation of outer sides of bent portions using a magnifier with a magnification of ×30. In the case where no crack was formed throughout a width of 25 mm of each test piece or in the case where at most five microcracks having a length of 0.2 μm or less were formed throughout a width of 25 mm of each test piece, the test piece was regarded as being free from cracks. The evaluation criterion for bendability was as follows: limit bending radius/thickness (R/t) 4.0.

(Hydrogen Analysis Method)

A strip-shaped plate having a long-axis length of 30 mm and a short-axis length of 5 mm was taken from the middle portion of each of the electrolytic zinc-based coated steel sheets in the width direction. The coating on the surfaces of the strip was completely removed with a handy router. Hydrogen analysis was performed with a thermal desorption spectroscopy system at a rate of temperature increase of 200° C./h. Note that the hydrogen analysis was performed immediately after the strip-shaped plate was taken and then the coating was removed. The cumulative amount of hydrogen released from a heating start temperature (25° C.) to 200° C. was measured and used as the amount of diffusible hydrogen in the steel.

3. Evaluation Result

Tables 3-1 to 3-4 present the evaluation results.

TABLE 3-1 Steel microstructure TM + B *2 Total Amount of Mechanical properties TM + in surface perimeter of diffusible hydrogen TS × Steel B *1 layer portion fine carbide *3 in steel TS El El No. grade % % μm/mm2 ppm by mass MPa % MPa · % R/t  1 A 97 87 67 0.03 1840 7.8 14352 3.1 Example  2 96 88 61 0.07 1830 7.7 14091 3.6 Example  3 97 88 64 0.06 1840 7.7 14168 3.3 Example  4 95 90 63 0.29 1810 7.6 13756 4.2 Comparative example  5 97 87 67 0.17 1820 7.8 14196 3.5 Example  6 97 92 66 0.16 1830 7.7 14091 3.2 Example  7 98 80 55 0.13 1840 7.7 14168 3.4 Example  8 96 77 48 0.18 1820 7.8 14196 4.1 Comparative example  9 B 93 88 60 0.19 1570 8.7 13659 3.5 Example 10 92 83 66 0.16 1560 8.7 13572 3.6 Example 11 93 84 55 0.20 1570 8.7 13659 3.3 Example 12 94 89 43 0.15 1580 8.7 13746 4.5 Comparative example 13 C 93 87 61 0.16 1580 8.6 13588 3.6 Example 14 93 85 64 0.09 1580 8.6 13588 3.2 Example 15 92 87 51 0.10 1570 8.7 13659 3.8 Example 16 93 78 45 0.07 1580 8.7 13746 4.7 Comparative example 17 D 97 91 64 0.02 1830 7.8 14274 3.6 Example 18 98 93 69 0.05 1840 7.7 14168 3.5 Example 19 98 81 52 0.08 1840 7.7 14168 3.8 Example 20 96 77 47 0.13 1820 7.8 14196 4.4 Comparative example 21 E 99 91 56 0.11 2020 7.4 14948 3.4 Example 22 99 93 55 0.18 2010 7.4 14874 3.7 Example 23 98 81 64 0.17 2000 7.4 14800 3.7 Example 24 99 77 58 0.10 2030 7.3 14819 4.5 Comparative example 25 F 97 89 52 0.18 1950 7.5 14625 3.4 Example 26 97 91 51 0.17 1950 7.5 14625 3.2 Example 27 98 89 53 0.18 1960 7.5 14700 3.3 Example 28 98 90 51 0.10 1960 7.4 14504 3.5 Example 29 G 96 86 68 0.18 1880 7.6 14288 3.2 Example 30 94 87 65 0.06 1860 7.7 14322 3.4 Example 31 91 84 67 0.10 1820 7.8 14196 3.6 Example 32 88 74 65 0.32 1740 7.9 13746 4.5 Comparative example *1 The total area percentage of martensite (TM) containing a carbide having an average particle size of 50 nm or less and bainite (B) containing a carbide having an average particle size of 50 nm or less in the entire microstructure. *2 The total area percentage of TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less in a region extending from a surface to a depth of ⅛ of the thickness of the sheet (surface layer portion). *3 The total perimeter of carbide particles having an average particle size of 50 nm or less in TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less present in the surface layer portion. Underlined values are outside the scope of the present invention.

TABLE 3-2 Steel microstructure TM + B *2 Total Amount of Mechanical properties TM + in surface perimeter of diffusible hydrogen TS × Steel B *1 layer portion fine carbide *3 in steel TS El El No. grade % % μm/mm2 ppm by mass MPa % MPa · % R/t 33 H 92 84 70 0.13 1400 9.4 13160 3.3 Example 34 91 84 68 0.13 1410 9.4 13254 3.4 Example 35 90 83 60 0.18 1360 9.6 13056 3.0 Example 36 84 79 61 0.24 1290 9.8 12642 4.2 Comparative example 37 I 92 76 48 0.15 1590 8.6 13674 4.8 Comparative example 38 92 81 51 0.05 1580 8.7 13746 3.2 Example 39 93 84 60 0.14 1600 8.6 13760 3.5 Example 40 92 85 53 0.11 1580 8.7 13746 3.7 Example 41 J 99 75 45 0.16 2150 7.1 15265 4.1 Comparative example 42 97 96 69 0.16 2160 7.1 15336 3.2 Example 43 97 96 58 0.09 2160 7.1 15336 3.3 Example 44 98 96 45 0.05 2140 7.1 15194 4.3 Comparative example 45 K 97 82 42 0.16 1850 7.7 14245 4.2 Comparative example 46 98 82 54 0.20 1860 7.7 14322 3.8 Example 47 97 83 66 0.09 1850 7.7 14245 3.3 Example 48 96 78 42 0.14 1830 7.8 14274 4.4 Comparative example 49 L 99 84 56 0.10 1960 7.5 14700 3.5 Example 50 99 82 64 0.11 1960 7.5 14700 3.6 Example 51 98 81 60 0.06 1980 7.4 14652 3.8 Example 52 98 68 41 0.19 1970 7.5 14775 4.1 Comparative example 53 M 98 93 62 0.09 1900 7.6 14440 3.1 Example 54 97 89 57 0.16 1890 7.6 14364 3.7 Example 55 99 82 54 0.17 1910 7.6 14516 3.4 Example 56 98 78 46 0.15 1900 7.6 14440 4.4 Comparative example 57 N 99 93 64 0.21 1910 7.4 14134 4.2 Comparative example 58 98 91 65 0.22 1880 7.5 14100 4.3 Comparative example 59 99 91 60 0.09 1890 7.6 14364 3.2 Example 60 99 92 68 0.06 1900 7.8 14820 3.0 Example *1 The total area percentage of martensite (TM) containing a carbide having an average particle size of 50 nm or less and bainite (B) containing a carbide having an average particle size of 50 nm or less in the entire microstructure. *2 The total area percentage of TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less in a region extending from a surface to a depth of ⅛ of the thickness of the sheet (surface layer portion). *3 The total perimeter of carbide particles having an average particle size of 50 nm or less in TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less present in the surface layer portion. Underlined values are outside the scope of the present invention.

TABLE 3-3 Steel microstructure Amount Mechanical properties TM + B *2 Total of diffusible TS × TM + in surface perimeter of hydrogen El Steel B*1 layer portion fine carbide *3 in steel TS El MPa · No. grade % % μm/mm2 ppm by mass MPa % % R/t 61 O 98 95 66 0.17 1960 7.5 14700 3.7 Example 62 99 91 61 0.11 1950 7.5 14625 3.0 Example 63 99 82 53 0.15 1940 7.5 14550 3.6 Example 64 99 78 45 0.09 1950 7.5 14625 4.2 Comparative example 65 P 92 82 60 0.03 1670 8.3 13861 3.7 Example 66 94 79 45 0.09 1690 8.4 14196 4.2 Comparative example 67 92 82 42 0.02 1670 8.3 13861 4.2 Comparative example 68 93 87 64 0.18 1680 8.2 13776 3.8 Example 69 Q 96 86 66 0.06 1830 7.8 14274 3.0 Example 70 95 78 43 0.07 1820 7.8 14196 4.3 Comparative example 71 97 91 49 0.06 1840 7.7 14168 4.3 Comparative example 72 97 88 67 0.06 1830 7.8 14274 3.0 Example 73 R 94 76 49 0.08 1750 8.0 14000 4.6 Comparative example 74 95 85 45 0.04 1760 8.0 14080 4.5 Comparative example 75 92 86 69 0.12 1710 8.2 14022 3.6 Example 76 93 84 60 0.04 1730 8.1 14013 3.9 Example 77 S 93 76 47 0.09 1760 8.0 14080 4.3 Comparative example 78 93 85 57 0.07 1750 8.0 14000 3.6 Example 79 94 79 48 0.18 1760 8.0 14080 4.1 Comparative example 80 92 86 46 0.06 1730 8.1 14013 4.2 Comparative example 81 T 94 89 62 0.05 1800 7.8 14040 3.3 Example 82 95 90 61 0.06 1810 7.8 14118 3.2 Example 83 93 79 48 0.01 1790 7.8 13962 4.1 Comparative example 85 U 96 91 63 0.16 1890 7.6 14364 3.3 Example 86 98 91 55 0.10 1920 7.6 14592 3.2 Example 87 96 89 67 0.15 1900 7.6 14440 3.0 Example 88 97 77 45 0.15 1900 7.6 14440 4.4 Comparative example 89 V 96 89 69 0.03 1840 7.7 14168 3.5 Example 90 95 81 53 0.16 1830 7.7 14091 3.2 Example 91 95 78 48 0.02 1830 7.7 14091 4.3 Comparative example 92 96 90 55 0.04 1840 7.7 14168 3.7 Example *1 The total area percentage of martensite (TM) containing a carbide having an average particle size of 50 nm or less and bainite (B) containing a carbide having an average particle size of 50 nm or less in the entire microstructure. *2 The total area percentage of TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less in a region extending from a surface to a depth of ⅛ of the thickness of the sheet (surface layer portion). *3 The total perimeter of carbide particles having an average particle size of 50 nm or less in TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less present in the surface layer portion. Underlined values are outside the scope of the present invention.

TABLE 3-4 Steel microstructure Amount Mechanical properties TM + B *2 Total of diffusible TS × TM + in surface perimeter of hydrogen El Steel B*1 layer portion fine carbide *3 in steel TS El MPa · No. grade % % μm/mm2 ppm by mass MPa % % R/t  93 W 99 91 59 0.14 2130 7.1 15123 3.4 Example  94 99 82 53 0.12 2110 7.2 15192 3.5 Example  95 99 78 45 0.10 2090 7.2 15048 4.4 Comparative example  96 99 96 65 0.03 2140 7.1 15194 3.5 Example  97 X 99 95 70 0.08 2060 7.3 15038 3.4 Example  98 99 90 70 0.02 2040 7.3 14892 3.4 Example  99 99 78 45 0.18 2050 7.3 14965 4.2 Comparative example 100 99 91 61 0.03 2040 7.3 14892 3.6 Example 101 Y 99 92 66 0.16 1930 7.5 14475 3.5 Example 102 99 77 43 0.06 1940 7.5 14550 4.3 Comparative example 103 99 89 68 0.16 1930 7.5 14475 3.0 Example 104 98 93 68 0.10 1920 7.6 14592 3.8 Example 105 Z 97 91 59 0.02 1840 7.7 14168 3.4 Example 106 96 88 55 0.13 1820 7.8 14196 3.5 Example 107 97 91 65 0.07 1830 7.7 14091 3.6 Example 108 95 76 46 0.11 1800 7.8 14040 4.4 Comparative example 109 AA 94 86 67 0.20 1800 7.8 14040 3.9 Example 110 96 88 57 0.14 1820 7.8 14196 3.6 Example 111 96 89 56 0.08 1820 7.8 14196 3.6 Example 112 95 77 49 0.16 1810 7.8 14118 4.2 Comparative example 113 AB 97 89 61 0.15 1820 7.8 14196 3.6 Example 114 96 91 45 0.15 1810 7.8 14118 4.3 Comparative example 115 95 86 61 0.03 1800 7.8 14040 3.0 Example 116 95 85 64 0.10 1800 7.8 14040 3.3 Example 117 AC 98 96 65 0.12 2230 6.5 14495 3.4 Comparative example 118 AD 83 74 67 0.24 1480 9.0 13320 4.4 Comparative example 119 AE 94 89 41 0.22 1770 7.9 13983 4.2 Comparative example 120 AF 93 78 45 0.05 1310 9.8 12838 4.4 Comparative example 121 AG 94 79 60 0.20 1770 7.9 13983 4.7 Comparative example 122 AH 93 78 67 0.03 1760 8.0 14080 4.4 Comparative example 123 AI 93 87 44 0.10 1700 8.2 13940 4.4 Comparative example 124 AJ 96 89 47 0.18 1910 7.6 14516 4.4 Comparative example 125 AK 98 92 48 0.16 1830 7.9 14457 4.1 Comparative example 126 AL 94 79 66 0.03 1700 8.2 13940 4.7 Comparative example *1 The total area percentage of martensite (TM) containing a carbide having an average particle size of 50 nm or less and bainite (B) containing a carbide having an average particle size of 50 nm or less in the entire microstructure. *2 The total area percentage of TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less in a region extending from a surface to a depth of ⅛ of the thickness of the sheet (surface layer portion). *3 The total perimeter of carbide particles having an average particle size of 50 nm or less in TM containing a carbide having an average particle size of 50 nm or less and B containing a carbide having an average particle size of 50 nm or less present in the surface layer portion. Underlined values are outside the scope of the present invention.

In the examples, a steel sheet satisfying TS 1,320 MPa, El≥7.0%, TS×El≥12,000, and R/t≤4.0 was rated acceptable and presented as “Example” in Tables 3-1 to 3-4. A steel sheet that does not satisfy at least one of TS 1,320 MPa, El≥7.0%, TS×El≥12,000, and R/t≤4.0 was rated unacceptable and presented as “Comparative example” in Tables 3-1 to 3-4. Underlines in Tables 1 to 3-4 indicate that the requirements, production conditions, and properties according to aspects of the present invention are not satisfied.

Claims

1-10. (canceled)

11. A high-ductility, high-strength electrolytic zinc-based coated steel sheet comprising an electrolytic zinc-based coating on a surface of a base steel sheet,

wherein the base steel sheet has a component composition containing, on a percent by mass basis,
C: 0.12% or more and 0.40% or less,
Si: 0.001% or more and 2.0% or less,
Mn: 1.7% or more and 5.0% or less,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.010% or more and 0.20% or less,
N: 0.010% or less, and
Sb: 0.002% or more and 0.10% or less, the balance being Fe and incidental impurities; and
a steel microstructure in which a total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, a total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ⅛ of a thickness of the base steel sheet, and a total perimeter of individual carbide particles having an average particle size of 50 nm or less in the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less present in the region is 50 μm/mm2 or more,
wherein an amount of diffusible hydrogen in steel is 0.20 ppm or less by mass.

12. The high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 11, wherein the component composition further contains, on a percent by mass basis, at least one selected from the group consisting of:

group A: B: 0.0002% or more and less than 0.0035%.
group B: one or two selected from Nb: 0.002% or more and 0.08% or less, and Ti: 0.002% or more and 0.12% or less.
group C: one or two selected from Cu: 0.005% or more and 1% or less, and Ni: 0.01% or more and 1% or less.
group D: one or two or more selected from Cr: 0.01% or more and 1.0% or less, Mo: 0.01% or more and less than 0.3%, V: 0.003% or more and 0.5% or less, Zr: 0.005% or more and 0.2% or less, and W: 0.005% or more and 0.2% or less.
group E: one or two or more selected from Ca: 0.0002% or more and 0.0030% or less, Ce: 0.0002% or more and 0.0030% or less, La: 0.0002% or more and 0.0030% or less, and Mg: 0.0002% or more and 0.0030% or less.
group F: Sn: 0.002% or more and 0.1% or less.

13. A method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet, comprising:

a hot-rolling step of hot-rolling a steel slab having the component composition described in claim 11 at a slab heating temperature of 1,200° C. or higher and a finish hot-rolling temperature of 840° C. or higher, performing cooling to a primary cooling stop temperature of 700° C. or lower at an average cooling rate of 40° C./s or more in a temperature range of the finish hot-rolling temperature to 700° C., performing cooling at an average cooling rate of 2° C./s or more in a temperature range of the primary cooling stop temperature to 650° C., performing cooling to a coiling temperature of 630° C. or lower, and performing coiling;
an annealing step of heating a steel sheet after the hot-rolling step to an annealing temperature equal to or higher than an AC3 point or performing heating to an annealing temperature equal to or higher than an AC3 point and performing soaking, performing cooling to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./s or more in a temperature range of the annealing temperature to 550° C., and performing holding at a holding temperature in a temperature range of 100° C. to 200° C. for 20 to 1,500 seconds; and
a coating treatment step of cooling the steel sheet after the annealing step to room temperature and subjecting the steel sheet to electrolytic zinc-based coating for an electroplating time of 300 seconds or less.

14. A method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet, comprising:

a hot-rolling step of hot-rolling a steel slab having the component composition described in claim 12 at a slab heating temperature of 1,200° C. or higher and a finish hot-rolling temperature of 840° C. or higher, performing cooling to a primary cooling stop temperature of 700° C. or lower at an average cooling rate of 40° C./s or more in a temperature range of the finish hot-rolling temperature to 700° C., performing cooling at an average cooling rate of 2° C./s or more in a temperature range of the primary cooling stop temperature to 650° C., performing cooling to a coiling temperature of 630° C. or lower, and performing coiling;
an annealing step of heating a steel sheet after the hot-rolling step to an annealing temperature equal to or higher than an AC3 point or performing heating to an annealing temperature equal to or higher than an AC3 point and performing soaking, performing cooling to a cooling stop temperature of 350° C. or lower at an average cooling rate of 3° C./s or more in a temperature range of the annealing temperature to 550° C., and performing holding at a holding temperature in a temperature range of 100° C. to 200° C. for 20 to 1,500 seconds; and
a coating treatment step of cooling the steel sheet after the annealing step to room temperature and subjecting the steel sheet to electrolytic zinc-based coating for an electroplating time of 300 seconds or less.

15. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 13, further comprising, after the hot-rolling step, a cold-rolling step of cold-rolling the steel sheet between the hot-rolling step and the annealing step.

16. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 14, further comprising, after the hot-rolling step, a cold-rolling step of cold-rolling the steel sheet between the hot-rolling step and the annealing step.

17. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 13, further comprising a tempering step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:

(T+273)(log t+4)≤2,700  (1)
where in formula (1), T is a holding temperature (° C.) in the tempering step, and t is the holding time (s) in the tempering step.

18. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 14, further comprising a tempering step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:

(T+273)(log t+4)≤2,700  (1)
where in formula (1), T is a holding temperature (° C.) in the tempering step, and t is the holding time (s) in the tempering step.

19. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 15, further comprising a tempering step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:

(T+273)(log t+4)≤2,700  (1)
where in formula (1), T is a holding temperature (° C.) in the tempering step, and t is the holding time (s) in the tempering step.

20. The method for producing a high-ductility, high-strength electrolytic zinc-based coated steel sheet according to claim 16, further comprising a tempering step of holding the steel sheet after the coating treatment step in a temperature range of 250° C. or lower for a holding time t that satisfies formula (1) below:

(T+273)(log t+4)≤2,700  (1)
where in formula (1), T is a holding temperature (° C.) in the tempering step, and t is the holding time (s) in the tempering step.
Patent History
Publication number: 20210324504
Type: Application
Filed: Aug 6, 2019
Publication Date: Oct 21, 2021
Applicant: JFE Steel Corporation (Tokyo)
Inventors: Takuya Hirashima (Chiyoda-ku, Tokyo), Shinjiro Kaneko (Chiyoda-ku, Tokyo)
Application Number: 17/285,166
Classifications
International Classification: C22C 38/06 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C21D 8/02 (20060101); C21D 9/46 (20060101); C22C 38/00 (20060101); C25D 3/22 (20060101);